Pulp is the raw material for the production of paper, paperboard, fiberboard and the like. In purified form, it is a source of cellulose for rayon, cellulose esters and other cellulosic products. Pulp is obtained from plant fiber such as wood, straw, bamboo and sugarcane residues. Wood is the source of 99% of the pulp fiber produced in the United States.
Dry wood consists of 40 to 50 percent cellulose, 15 to 25 percent other polysaccharides known as hemicelluloses, 20-30 percent lignin, a biopolymer which acts as a matrix for the cellulose fibers, and 5 percent of other substances such as mineral salts, sugars, fat, resin and protein. Lignin is composed primarily of methoxylated phenyl propane monomeric units interconnected by a variety of stable C--C and ether linkages. The lignin of conifers is apparently an oxidative polymerization product of coniferyl alcohol [3-(3'-methoxy-4'-hydroxyphenyl)allyl alcohol], while the lignin of deciduous trees appears to be derived from coniferyl alcohol and sinapyl alcohol [3-(3',5'-dimethoxy-4'-hydroxyphenyl)allyl alcohol].
Wood pulp is manufactured by dissolving the lignin with hot solutions of (1) sodium hydroxide, (2) calcium, magnesium, or ammonium bisulfite, or (3) a mixture of sodium hydroxide and sodium sulfide (made from lime and reduced sodium sulfate). The products, known as soda pulp, sulfite pulp or sulfate (kraft) pulp, respectively, consist of impure cellulose.
Under acid or alkaline pulping conditions, condensation ractions take place homolytically within the lignin polymer and very likely occur between lignin and carbohydrates such as hemicelluloses. Formaldehyde is also generated from the lignin residues in the presence of sodium hydroxide which can cause condensation and cross-linking of the phenylpropyl moieties. These reactions are undesirable in delignification. Chemical pulping processes are often conducted at high temperatures and pressures. These operating parameters are energy intensive and require costly equipment. Furthermore, kraft pulping yields volatile malodorous sulfur compounds such as hydrogen sulfide, methyl mercaptan and dimethyl disulfide which are hazardous to human health and to the environment.
Bleaching processes may also be applied to the crude pulp, in order to complete the delignification process and remove pitch. The bleaching reagents are mostly oxidative. One bleaching method involves an initial chlorination of the lignin under acidic conditions, followed by alkaline hydrolysis and extraction of the chlorinated lignin. Further brightening of the pulp is accomplished with chlorine dioxide. Bleach plant effluents contain polymeric lignin degradation products which are highly colored, along with corrosive chloride ion. These effluents are resistant to current bacteria-based biological wastewater treatment processes and must be decolorized via expensive filtration or precipitation steps prior to their discharge into the environment.
Therefore, a need exists for methods to delignify plant fiber or pulp which minimize the undesirable condensation reactions caused by the presently-employed chemical pulping and bleaching methods. A further need exists for lignin-degrading pulp bleaching methods which eliminate the damage to cellulosic fibers which can be caused by chlorinebased oxidizing agents. A further need exists for methods to degrade lignin which are energy efficient, and which eliminate the environmental release of polluting lignin-derived chlorinated aromatic compounds.